Nitrile group-containing highly saturated copolymer rubber, typified by hydrogenated acrylonitrile-butadiene copolymer rubber, is superior in heat resistance, compression set resistance, and fuel oil resistance, so is being used around engines in large amounts as O-rings, gaskets, packings, and other seal members.
On the other hand, such nitrile group-containing highly saturated copolymer rubber has a problem that so-called “mold fouling” is occurred when being cross-linked using a mold in a molding process. That is, when cross-linked using a mold in an injection molding or other molding process, residue gradually deposits and builds up at a repeatedly used mold. As a result, there is the problem that even the obtained shaped product is fouled and a shaped product with excellent surface conditions can no longer be obtained. Therefore, the mold has to be cleaned at periodic intervals. This cleaning requires tremendous time and cost and becomes a major cause lowering productivity. Further, if using a release agent as a countermeasure to this, control of the amount of use of the release agent is difficult. Further, this ends up causing weld defect and other defects.
As opposed to this, for example, Patent Document 1 discloses, for improving the release property and mold fouling resistance, using a hydrogenated nitrile butadiene rubber composition comprised of hydrogenated nitrile butadiene rubber having a Mooney viscosity (ML1+4, 100° C.) of 57.5 to 150 to which a fatty acid metal salt of zinc stearate or zinc laurate and a cross-linking agent of an organic peroxide have been added. However, in the method of this document, there were the inconveniences that zinc stearate or zinc laurate had to be newly added later and the effect of improvement of the mold fouling resistance was not sufficient.    Patent Document 1: Japanese Patent Publication (A) No. 2003-226780